The solid desiccants used in air treatment or liquid absorption systems are primarily inorganic (silica gel, molecular sieves, etc). They take the form of fine powders which must be bonded to a rigid substrate. There are a number of techniques for depositing these desiccants, some of which have been patented. Examples include patents filed in the United States under U.S. Pat. Nos. 3,338,034; 4,769,053; 5,052,188; 5,120,694; 5,496,397; and 5,542,968.
U.S. Pat. No. 5,542,968 describes a method which involves mixing the desiccant powder with fibres in a solution containing a binder and fire retardants, among other ingredients. A manufacturing process borrowed from the paper industry is then used to produce sheets of this compound. Canadian Patent No. 1,285,931 uses a technique which consists of coating a metallic substrate with a mixture consisting primarily of an inorganic desiccant and a heat-curable binder or adhesive in a solvent. The powder is then bonded to the substrate by heating the article. U.S. Pat. No. 4,172,164 describes the use of a solvent to partially dissolve the thermoplastic substrate, leaving the polymer particles imbedded in it following evaporation of the solvent. These techniques have the disadvantage of inhibiting to some extent the absorption of water by the desiccant powder, which may deliquesce and become detached under conditions of actual use.
Another category of articles relates to water absorption in diapers or sanitary napkins. The absorbent materials used in these applications are polymers capable of absorbing up to hundreds of times their own weight in water and are hence referred to as superabsorbents. U.S. Pat. Nos. 3,699,103 and 3,810,468 describe a method which consists of spreading a copolymer of acrylic acid and acrylamide, in powder form, on a fibrous material. This material is exposed to steam to swell the particles of powder, then compressed and dried to bond the polymer to the fibres. One of the disadvantages of this method is the fact that the polymer detaches from the fibres after absorbing the liquid and swelling. Another method described in U.S. Pat. No. 3,005,456 consists of treating the fibres with chloroacetic acid to permit the attachment of carboxymethyl groups for absorbency. This technique uses chloroacetic acid, a very expensive product, in a propanol solution. In addition, the absorptive capacity of the fibres is considered insufficient. Finally, another method described in U.S. Pat. No. 5,026,596 consists of producing a water absorbent polymer coated article which has excellent water absorption and swelling properties. This patent is the result of continual improvement of the techniques described in Japanese Patents Nos. 50-82143/1975, 55-84304/1980 and 58-84804/1983. These applications involve large quantities of polymer and, once it is swollen by the absorbed liquid, the structure of the article deteriorates and disintegrates.